There have been numerous reports across the state of soybean sudden death syndrome (SDS) showing up in certain fields since August. SDS, which can be a devastating disease, is caused by the soil fungus Fusarium virguliforme (Fv). Fv is actually a soilborne fungus that attacks roots early in the season and later causes a root rot. Infection is favored by cool soils with good moisture; thus, this spring was perfect for infection in most full-season fields. As plants go into the reproductive stages, the causal fungus produces a plant toxin that causes above ground foliar symptoms of yellowing and tissue death between the veins (see below picture), and later defoliation. If the disease comes into the field during the early pod development stages, and enough of the field is impacted, SDS can result in almost total yield loss. This is, however, an extremely rare event in KY. More typically, the disease causes significant yield losses in spots in a field - usually the lowest portions where soil moisture is greatest - but the majority of the field has little to know yield impact. That is, most plants either escape disease altogether or symptoms come in after mid to late -pod fill. Plants showing late symptoms will generally yield very well. Thus, the appearance of symptoms is not necessarily a good indicator of yield loss potential. Timing of symptoms is everything when it comes to SDS.
SDS is variety dependent. Many cultivars are available with respectable resistance to SDS. Some are fully susceptible. That is why fields with a history of SDS should always be planted to a resistant cultivar. SDS is also favored by early planting. This is because infection by the Fv is greatest when soil is cool and wet. Generally, very early planting dates meet these requirements for infection in most years. That is just one reason why growers take a great risk when planting soybean in late April. For exactly the opposite reason, double crop soybeans rarely show significant levels of SDS. This is because doublecrop beans are planted in late June to early July. Soil temperatures and moisture levels are rarely highly conducive to infection by the SDS pathogen during late-planted situations.
If you have a field that you think has SDS, the most desirable way forward is to send a sample to one of our two Plant Disease Diagnostic Laboratories for confirmation. Foliar symptoms can be mimicked by other diseases and even certain insects, so confirmation based on foliar symptoms alone may not yield an accurate diagnosis. But the foliar symptoms coupled with rotten roots and a milky brown stem discoloration (when a longitudinal cut is made into the stem) are the "trifecta" of symptoms for SDS. No other soybean disease will show those three symptoms at the same time.
Note: Foliar fungicides do not have a direct effect on SDS. However, in rare instances a very limited (favorable) indirect effect may be evidenced by a slight reduction in SDS foliar symptom severity. This indirect effect is unpredictable, but when it occurs is related to stress reduction following the application of certain fungicides, especially strobilurin fungicides. Currently available seed treatment fungicides are ineffective at reducing infection by the SDS pathogen, Fv. Beginning next year, a seed treatment product may be available that is capable of reducing infection by Fv (there by reducing SDS). More on this potential breakthrough later.
Source : grainblog.uky.edu